Fixes #107 Hook on "deploy function" command

[HyperBrain]

Fixes #107: This fix hooks the webpack plugin onto the Serverless "deploy function" command. Currently the plugin will webpack the service as it would be done with a regular deploy. Then the single selected function is deployed by Serverless.
This makes the whole thing work. However there could be some improvements done later. Maybe this.options.f can be used in this case to restrict the webpack run to create only the selected function entry point. But finding that would not be trivial.
I suggest to approve this solution on the first hand and handle the rest as improvement.

[mnightingale]

This doesn't seem to work when you have multiple entry/outputs in webpack.config.js similar to using the example config from https://webpack.github.io/docs/multiple-entry-points.html

$ sls deploy function -f x
Serverless: Deploying function: x...
Serverless: Bundling with Webpack...
ts-loader: Using typescript@2.2.1 and /Users/XYZ/tsconfig.json
Time: 1248ms
Asset Size Chunks Chunk Names
x.js 620 kB 0 [emitted] x
y.js 4.5 kB 1 [emitted] y
z.js 3.21 kB 2 [emitted] z
Serverless: Packaging function: x...
path.js:7
throw new TypeError('Path must be a string. Received ' + inspect(path));
^

TypeError: Path must be a string. Received undefined
at assertPath (path.js:7:11)
at Object.basename (path.js:1355:5)
at fse.copy (/Users/XYZ/node_modules/serverless-webpack/lib/cleanup.js:37:22)
at doneOne (/Users/XYZ/node_modules/fs-extra/lib/copy/ncp.js:237:40)
at /Users/XYZ/node_modules/fs-extra/lib/copy/ncp.js:122:11
at /Users/XYZ/node_modules/graceful-fs/polyfills.js:239:18
at FSReqWrap.oncomplete (fs.js:112:15)

[martinmicunda]

@mnightingale The error is caused by after:deploy:function:packageFunction if you remove that part of the code it works however I found another problem when you run sls deploy function -f test it will create .serverless folder under .webpack directory so structure looks like this:

|-.webpack
    |-.serverless
         |-test.zip

and this cause follow error as the serverless expact zip file to be under .serverless in root folder not in .webpack:

ENOENT: no such file or directory, open '<-your-path->/.serverless/test.zip'

My current solution is below but is not ideal however it still better then deploy cloudformation during development:

sls package -v && sls deploy function -f test -v

[franciscocpg]

Hello @martinmicunda .
This PR #130 should fix it.

[HyperBrain]

Superseded by #130